1. Field of the Invention
The present invention relates to a reinforcing member for vehicle center pillar, and especially it relates to an improvement of a reinforcing member constructed using a plurality of steel plates that are different in tensile strength.
2. Description of the Prior Art
To secure safety in a lateral collision of an automobile, a center pillar is reinforced. That is, a reinforcing member (reinforcement) is fitted and fixed integrally by welding to an interior of a center pillar constructed by a center pillar outer panel and a center pillar inner panel to have a closed cross-section. Due to characteristics of cross-sectional shape, such a center pillar reinforcing member is low in the section modulus (corresponding to a cross section second moment) at an upper part, and is high in the section modulus at a lower part. In view of this, the upper part is constructed by a thick plate and the lower part is constructed by a thin plate. However recently, to satisfy needs for improvement of lateral collision performance and for realizing light weight i.e. weight saving to improve fuel consumption, it is proposed to form the upper part by an ultra-high tensile steel plate.
An example of such a reinforcing member is described in Patent Document 1 (Japanese Unexamined Patent Publication No. 2006-281964) and is shown in FIG. 3. FIG. 3 shows a reinforcing member for center pillar at a left side face of a vehicle, with FIG. 3A being a front view as viewed from a left side of the vehicle, and FIG. 3B being a right side view of FIG. 3A, i.e. a side view as viewed from a rear side of the vehicle. The reinforcing member is constructed by two members, i.e. an upper member 100 and a lower member 102 that are bonded by welding integrally.
The upper member 100 is constructed by an ultra-high tensile steel plate with tensile strength of 980 MPa class, and the lower member 102 is constructed by a high tensile steel plate with a tensile strength of 440 MPa class. With suppressing bending into L-shape and buckling of the upper part of the center pillar upon lateral collision, a predetermined safety performance is secured, and with employing the thin and lightweight upper member 100 constructed by the ultra-high tensile steel plate fuel consumption is improved.
A center pillar reinforcing member described in Patent Document 2 (KR Publication No. 20040001152) is divided into three members, i.e. an upper portion a central portion, and a lower portion. By selecting the plate thickness of central reinforcing member thicker than those of the upper and lower reinforcing member members, deformation of the central portion is suppressed to thereby improve safety performance.
However, with the center pillar reinforcing member constructed by the two members i.e. the upper member and the lower member that are different in tensile strength as in Patent Document 1, non-uniform residual stress is generated readily in a product interior at the upper part constructed by the ultra-high tensile steel plate. Thus, deforming amounts due to camber recovery that accompanies elastic recovery and twisting become large, which lowers shape accuracy (see FIG. 3). This makes pressing for securing the predetermined shape accuracy in the reinforcing member difficult.
Also, at a roof side T-shaped portion of the reinforcing member i.e. at an upper T-shaped portion connected to a roof side rail, defects such as cracking occur readily in a product cross-section, due to poor pressing forming property such as drawing property. Moreover, because of inclination of the upper part of the reinforcing member toward an inner side of the vehicle, an angle thereof changes compared with a general pressing direction, so that the door frame corner portions being difficult in the outer trimming tend to generate burr readily. Lowered yield and productivity due to generation of such cracks and burrs, the employment of expensive ultra-high tensile steel, and the abovementioned difficulty of pressing, cause a large increase of manufacturing cost.
Meanwhile, when with the reinforcing member being divided into three portions, the plate thickness of the central reinforcing member is made thick as in Patent Document 2, weight increases as increase of the plate thickness. Moreover, strength and rigidity of the upper reinforcing member resulted from the small section modulus and thin plate thickness are relatively lowered, which worsens balance of strength and rigidity of the reinforcing member as a whole. For this reason, an adequate safety performance increasing effect of the reinforcing member cannot be obtained necessarily.